Semiconductor micro-resonator for monitoring an optical device

ABSTRACT

An optical device includes an optical waveguide through which light propagates and a micro-resonator structure including an optical sensor. The micro-resonator is configured to resonate at a wavelength of light that may be transmitted through the optical waveguide. When light at that wavelength is transmitted through the optical waveguide, it resonates in the resonator and is detected by the optical sensor to produce an electrical signal. The optical resonator may be a micro-cylinder, disc or ring resonator and may be coupled to the waveguide via evanescent coupling or leaky-mode coupling. Multiple resonators may be implemented proximate to the waveguide to allow multiple wavelengths to be detected. When the waveguide is coupled to a tunable laser, signals provided by the optical sensor may be used to tune the wavelength of the laser.

[0001] This application is a divisional application of U.S. applicationSer. No. 10/245,075, filed Sep. 16, 2002, which claims benefit ofpriority from U.S. provisional application No. 60/375,881 filed Apr. 26,2002 the contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002] The present invention relates to optical monitoring devices andin particular to a micro-resonator which is coupled to an optical deviceto produce an electronic signal when the wavelength of the lightprovided in the optical device is within the resonance bandwidth of themicro-resonator.

[0003] Optical monitors are used with many optical devices. They may beused, for example, to determine the wavelength and/or optical power oflight produced by a semiconductor laser in order to tune the laser.Optical monitors may also be used to determine the losses in an opticalsystem, such as a electro-absorptive modulator or semiconductor opticalamplifier, by measuring both the input energy and output energy of thesystem either over a broad band of wavelengths or in a specific narrowband. In the materials that follow, it is assumed that the light to bemeasured is propagating through a waveguide or is generated in anoptical gain medium. In the material that follows, the term “waveguide”includes both traditional waveguides and gain media. Furthermore, theterm light is used to indicate any radiation that may be transmitted viaan optical waveguide.

[0004] An important use for optical monitoring systems is in tuningcommunications lasers. Communications lasers operating in a densewavelength division multiplexing (DWDM) system are desirably especiallyfinely tuned to be able to provide the closely spaced channels definedfor this standard. Exemplary channels for a DWDM system are defined asν_(n)=ν₀±ndν, where ν₀ is the central optical frequency, (e.g. 193.1THz) and dν is the channel spacing (e.g. 100 GHz or of 50 GHz).

[0005] Typical semiconductor lasers are able to be tuned in a range of30-40 nm while maintaining acceptable power levels. Tunablesemiconductor lasers may be a distributed feedback (DFB) laser, adistributed Bragg reflector (DBR) laser or other laser that usesdistributed mirrors. Tunable lasers may also be more conventional lasershaving a resonant cavity that includes at least one Fabry-Perot cavityas a reflector. In each of these lasers, the resonant wavelength may betuned by electrically or thermally adjusting the “optical length”between the reflectors. The optical length may be adjusted by changingthe actual length and/or the index of refraction of the material betweenthe reflectors. Although not explicitly described herein, pressure, asapplied by one or more piezoelectric elements, may also be used toadjust the index of refraction.

[0006] A typical laser tuning system couples the light provided by thelaser is coupled to a waveguide. A portion of the light travelingthrough the waveguide is tapped, for example, by splicing an opticalfiber through the cladding of the waveguide. This tapped light may beapplied to one or more optical filters that separate light having aparticular wavelength and then to an optical sensor, such as aphotodiode. The light tapped from the waveguide may also be directlydetected by an optical sensor to determine the power level of the laser.Splicing optical fibers to the output fiber to tap the light may causeundesirable power loss or scattering of light that may result inincreased noise or feedback into the laser.

[0007] The signals provided by the optical sensors may be applied tocontrol circuitry that adjusts the temperature of the laser or itsreflectors or adjusts an electrical potential applied to the reflectors.This circuitry changes the resonant wavelength of the laser to center itwithin a predetermined communications channel.

[0008] Thus, considerable circuitry, separate from the semiconductorlaser, is typically used to tune the laser.

SUMMARY OF THE INVENTION

[0009] The present invention is embodied in an optical structureincluding an optical waveguide through which light propagates and amicro-resonator structure, configured to receive light from thewaveguide and including an optical sensor. The micro-resonator isconfigured to resonate at a wavelength of light that may be transmittedthrough the optical waveguide. When light at that wavelength ispropagating through the optical waveguide, it resonates in the resonatorand is detected by the optical sensor to produce an electrical signal.

[0010] According to one aspect of the invention, the micro-resonatorstructure is formed as an optical sensor.

[0011] According to another aspect of the invention, the micro-resonatoris a micro-cylinder.

[0012] According to yet another aspect of the invention, themicro-resonator is positioned with respect to the optical waveguide sothat light propagating through the waveguide is received by themicro-resonator via evanescent coupling.

[0013] According to yet another aspect of the invention, themicro-resonator is positioned to intersect a portion of the opticalwaveguide so that light propagating through the waveguide is received bythe micro-resonator through leaky-mode coupling.

[0014] According to another aspect of the invention, the device includesa plurality of micro-resonators, each having a respectively differentresonant wavelength so that light having multiple different wavelengthsmay be detected.

[0015] According to another aspect of the invention, the waveguide iscoupled to receive light from a tunable laser and the electrical signalprovided by the micro-resonator provides a feedback signal to tune thelaser.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The invention is best understood from the following detaileddescription when read in connection with the accompanying drawing. It isemphasized that, according to common practice, the various features ofthe drawing are not to scale. On the contrary, the dimensions of thevarious features are arbitrarily expanded or reduced for clarity.Included in the drawing are the following figures:

[0017]FIGS. 1A, 1B and 1C are a side-plan view, front-plan view and topplan view of an optical device which includes a first embodiment of thesubject invention.

[0018]FIG. 2 is a block diagram of a laser wavelength control circuitwhich includes an embodiment of the subject invention.

[0019]FIGS. 3A, 3B and 3C are a side-plan view, front-plan view and topplan view of an optical device that includes a second embodiment of thesubject invention.

[0020]FIGS. 4A, 4B and 4C are a side-plan view, front-plan view and topplan view of an optical device including a third embodiment of thesubject invention.

[0021]FIGS. 5A, 5B and 5C are a side-plan view, front-plan view and topplan view of an optical device comprising a fourth embodiment of thesubject invention.

[0022]FIGS. 6A, 6B and 6C are a side-plan view, front-plan view and topplan view of an optical device which includes a fifth embodiment of thesubject invention.

[0023]FIGS. 7A, 7B and 7C are a side-plan view, front-plan view and topplan view of an optical device that includes a sixth embodiment of thesubject invention.

[0024]FIGS. 8A, 8B and 8C are a side-plan view, front-plan view and topplan view of an optical device including a seventh embodiment of thesubject invention.

[0025]FIGS. 9A, 9B and 9C are a side-plan view, front-plan view and topplan view of an optical device which includes an eighth embodiment ofthe subject invention.

[0026]FIGS. 10A, 10B and 10C are a side-plan view, front-plan view andtop plan view of an optical device that includes a ninth embodiment ofthe subject invention.

[0027]FIGS. 11A, 11B and 11C are a side-plan view, front-plan view andtop plan view of an optical device comprising a tenth embodiment of thesubject invention.

[0028]FIGS. 12A, 12B and 12C are a side-plan view, front-plan view andtop plan view of an optical device that includes an eleventh embodimentof the subject invention.

[0029]FIGS. 13A, 13B and 13C are a side-plan view, front-plan view andtop plan view of an optical device which comprises a twelfth embodimentof the subject invention.

[0030]FIG. 14 is a graph of wavelength versus amplitude that is usefulto describe the operation of the embodiment of the invention shown inFIGS. 13A, 13B and 13C.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

[0031] One exemplary embodiment of the present invention, illustrated inFIGS. 1A, 1B and 1C, is monitored waveguide device 100 including awaveguide 113 and a micro-cylinder 112. FIG. 1A is a side plan drawing,FIG. 1B is a front plan drawing and FIG. 1C is a top plan drawing of theof device 100. The exemplary device includes a substrate 110, wave guidelayer 118 and cladding layer 116. Waveguide material 118 desirablyexhibits low optical loss and a relatively high index of refraction inthe desired wavelength band. Although, in the devices described below,the waveguide material 118 may be various III/V materials, such as InP,GaAs, AlGaAs, or InGaAsP, other possible materials choices arecontemplated. Possible alternatives for the waveguide material 118include: doped silica (similar to optical fibers); silicon; germanium;and dielectric materials such as SiO₂ and SiN, which have low opticalloss characteristics for the desired wavelength band.

[0032] Cladding layer 116 and substrate 110 desirably have a lower indexof refraction than the waveguide material 118 and act as claddinglayers. The materials for these layers may desirably be selected from arelated family of materials to the material used to form waveguide layer118. Such a material selection may minimize scattering at the boundariesbetween layers and may also improve the quality of crystalline materialsby reducing lattice mismatches between layers. For example, thesubstrate 110 and the cladding layer 116 may be formed from InP, whilethe waveguide layer 118 may be formed from different alloys of, forexample, InGaAsP, layered to form one or more quantum well structures.

[0033] Additionally, waveguide layer 118 may contain a number ofsub-layers, forming a quantum well structure within this layer. Aquantum well waveguide structure may be desirable to increase thetunability of the index of refraction within the waveguide. Thisstructure may include a single quantum well, multiple quantum wells, orseparate confinement layers. Substrate layer 110 and cladding layer 116may also contain a plurality of sub-layers. In some of the embodiments,described below, the cladding layer 116 extends around the waveguide onthe sides as well as over the top of waveguide 118, as shown, forexample, in FIG. 5A. When the cladding layer 116 extends around thesides of the waveguide, it is desirably a dielectric material so that itdoes not interfere with the electrical operation of the waveguide 118when, for example, the waveguide is an active optical device, such as anoptical gain medium.

[0034] In addition to serving as a cladding layer, cladding layer 116may also desirably function as the p-type material of a P-I-N structurewith the substrate 110 forming the n-type material and the waveguidematerial 118 forming the intrinsic or i-type material.

[0035] The exemplary monitored waveguide device 100 also includes amicro-cylinder 112 which may be formed from the same material as thewaveguide 113. The exemplary embodiment, micro-cylinder 112 includes aresonator layer that may be formed from the waveguide material 118, acladding layer 116 and an electrical contact 114, that may be formedfrom a metal, such as Al, Au, Ag, Cu, Ni, Ti, W or a combination thereofor of another conductor such as polysilicon or polyaniline. Theexemplary micro-cylinders used in the subject invention are configuredto sense photocurrents induced by light near the desired resonantwavelength. In the exemplary embodiments of the invention, this may beachieved by several methods. First, the waveguide layer from which themicro-cylinder resonator is formed may include a quantum well structure.Photocurrents in a quantum well structure may be detected using asubstrate contact and a contact coupled to the cladding layer 116.Second, the micro-cylinder resonator may be formed from a bulk materialand the photocurrent may be sensed directly as in a photodiode or as achange in the conductance of the material as a photoresistor. Third, themicro-cylinder resonator may be formed from a dielectric material butpositioned proximate to a photodetector (not shown) such that when themicro-cylinder receives light at the resonant frequency, the resonantlight energy in the micro-cylinder is detected by the photodetector. Forexample, the photodetector may be a photodiode formed in the substrate110 and the micro-resonator may be formed immediately above thephotodiode.

[0036] The micro-cylinder resonators used in the present inventiondesirably operate in a whispering gallery mode. The circumference of thecylinder is ideally equal to an integral multiple of the wavelength atwhich the device is to be in resonance. Therefore, the circumference ofthe cylinder determines the resonant wavelengths of the cavity, as wellas the free spectral range, or wavelength difference between resonantwhispering gallery modes. If the cylinder is large enough then the freespectral range may be smaller than the useful wavelength range of thewaveguide. Therefore it may be desirable to size the micro-cylinder sothat its free spectral range (FSR) matches the desired channel spacing.It is contemplated, however, that micro-cylinders having FSRs that arelarger or smaller than the channel spacing may be used.

[0037] The minimum size of the micro-cylinder resonators is determinedby acceptable losses. As the size or the cylinder decreases, thecurvature of the light path increases. Increased curvature of the lightpath increases cavity losses in the micro-resonator. Increasing theindex of refraction of the micro-resonator material may decrease thesecavity losses. Therefore, it may be desirable to form the micro-cylinderfrom materials that have a relatively high index of refraction. Also, toassist with the formation of curved surfaces of the resonator it may bedesirable for the resonator material to be formed using dry etchant oran etchant having a relatively high viscosity so as to avoid producingpolygonal features that may result from preferential etching alongcrystal planes which may cause the light to propagate in modes otherthan the whispering gallery mode. By using sub-micron semiconductorfabrication techniques to improve surface smoothness, high finesse, lowloss, micro-cylinder resonators may be fabricated. These resonators mayeffectively select one wavelength channel out of a large number ofwavelength channels.

[0038] For small discs that exhibit low losses, the resonances may beexpressed by equation (1), $\begin{matrix}{{\lambda_{M}^{N} = \frac{2\quad \pi \quad n}{X_{N}^{N}}},} & (1)\end{matrix}$

[0039] where, n is the refractive index and X_(M) ^(N) is the Nth zeroof the Bessel function J of order M (J_(M)). For whispering gallerymodes, N=1.

[0040] For large discs, R>>λ₀ of operation, resonances may be expressedby equation (2). $\begin{matrix}{{\lambda_{m} = {\left( \frac{4\quad \pi \quad R}{m} \right)n}},{m = 1},2,\ldots} & (2)\end{matrix}$

[0041] Centered at λ₀<<R, the free spectral range is given by equation(3), $\begin{matrix}{{FSR} = {\left( \frac{1}{4\quad \pi} \right){\frac{\left( \frac{\lambda_{0}}{n} \right)^{2}}{R}.}}} & (3)\end{matrix}$

[0042] For λ₀≈1550 nm, n=3.2 and a Γ=0.4 nm is achieved with R=50 μm.For 50 μm discs the resonate peaks are separated by 0.4 nm. In order toshift the resonance peaks a voltage or current is used to bias thedevice and change the index of refraction by an amount Δλ_(m), given byequation (4). $\begin{matrix}{{\Delta \quad \lambda_{m}} = {\left( \frac{4\quad \pi \quad R}{n} \right)\Delta \quad n}} & (4)\end{matrix}$

[0043] With the maximum feasible Δn change being 0.01. Appropriatetuning of the resonant peaks may be easily achieved with Anapproximately equal to 0.001, as shown in equations (5) and (6).$\begin{matrix}{{\frac{\Delta \quad \lambda_{m}}{\lambda_{m}} = {{\frac{\Delta \quad n}{n} \approx \frac{0.01}{3.2}} = 0.03}},} & (5)\end{matrix}$

 Δλ_(m)|₁₅₅₀≈0.5 nm.  (6)

[0044] The micro-cylinder resonators of some embodiments of the presentinvention are designed to selectively couple to evanescent opticalfields that surround the waveguides. An evanescent optical field is theportion of the optical field of the light being guided by a waveguidethat extends beyond the physical surface of the waveguide. The intensityof the evanescent optical field decays exponentially with distanceoutside the physical surface of the waveguide. The high finesse of themicro-cylinder resonators desirably employed in one embodiment of thepresent invention allows even relatively small evanescent optical fieldintensities to couple readily detectable signals from the waveguide 113to the micro-cylinder 112, but only in relatively narrow ranges aboutthe resonant wavelengths of the micro-cylinder 112. Additionally, insome embodiments of the invention, this use of evanescent couplingallows relatively small signals to be transferred from the waveguide 113without physical contact between the waveguide 113 and themicro-cylinder resonator 112. Physical contact between themicro-cylinder results in scattering and may introduce undesirableadditional modes in the waveguide 113. The use of evanescent coupling ofthe high-finesse micro-resonator 112 to the waveguide 113 allows thedetection of tuning signals that are strongly wavelength dependent withminimal disturbance of the optical signal.

[0045] The exemplary monitored waveguide devices described below may beused to tune communications lasers for DWDM communications systems. Inthis environment, the high finesse of the resonant micro-cylinder 112may be undesirable as it may not receive light from the laser when it iseven slightly detuned from its ideal wavelength. Accordingly, in some ofthe embodiments described below, features are introduced that decreasethe finesse of the micro-cylinder, allowing light in a broader range ofwavelengths to be coupled to the micro-cylinder. Decreasing the finessemay also increase the free spectral range of the micro-cylinder. Asdescribed below, this may be useful for tuning a laser over a wide bandof wavelengths.

[0046] Although the device 113 is described as a waveguide, it may alsobe an active optical component, such as a laser, an optical amplifier,an optical attenuator or an electro-absorptive modulator (EAM). For anyof these devices, the micro-cylinder resonator 112 may be placed in thesame position with respect to the active device as it is placed withrespect to the waveguides described below.

[0047]FIG. 2 is a block diagram of an exemplary laser tuning circuitthat may be used with a monitor according to the present invention. Theexemplary circuit comprises a tunable laser 210 that includes awaveguide (not shown) and micro-cylinder (not shown) according to thepresent invention. The micro-cylinder is coupled to receive an optionalmicro-cylinder bias signal 212, that may be used to change therefractive index of the micro-cylinder and, so, change its resonantfrequency. The micro-cylinder is also coupled to provide a signal, thatis proportional to the induced photocurrent, to a controller 214. Thecontrol circuit may also receive an optional output signal PM from thelaser 210 indicating the instantaneous power level of the light beingemitted by the laser. Exemplary power monitor devices are describedbelow. The controller 214 provides an output signal λ CONT which is usedto control the wavelength of the laser. In the exemplary embodiments ofthe invention described below, the laser is a semiconductor laser thatmay be tuned thermally, electrically or by a combination of thermal andelectrical stimulus. Thermal stimulus may be applied, for example, usinga thermo-electric (TE) cooler (not shown). Electrical stimulus may beapplied, for example, to one or more Fabry-Perot reflectors (not shown)or to one or more passive dynamic feedback sections (not shown) of thelasers to change their refractive index and thus to change thewavelength of the reflected light and, thus, the wavelength of the lightproduced by the laser.

[0048] Referring to FIGS. 1A, 1B and 1C, the exemplary micro-cylinder112 is a relatively high-finesse device and is positioned in closeproximity to the waveguide 113. In this embodiment, the micro-cylinder112 is configured to resonate at a desired wavelength of the laser lightprovided via the waveguide 113. When the laser light is at the desiredfrequency, the sensed photocurrent provided by the resonator is at arelatively high level. When the wavelength of the light shifts away fromthe desired wavelength, the sensed photocurrent decreases to arelatively low level. Because the micro-cylinder 112 a relativelyhigh-finesse device, it has resonant peaks with a very narrow widthcompared to the free-spectral range and it is desirably placed in closeproximity to the waveguide 113. Accordingly, its dimensions and index ofrefraction are carefully controlled to produce a photocurrent at thedesired wavelength. The criticality of the dimensions of themicro-cylinder 112 may be reduced if, as in the exemplary embodiment ofthe invention, the micro-cylinder includes a quantum well structure orother structure (e.g. bulk semiconductor material) having an index ofrefraction that may be adjusted in response to a pressure or temperaturechange or to an electrical current or voltage applied between thecontact 114 and the substrate 110. A change in the refractive index ofthe micro-cylinder results in a change in its resonant frequency. Thus,in this exemplary embodiment of the invention, shown in FIGS. 1A, 1B and1C, both the laser and the micro-cylinder monitor may be tuned.

[0049]FIGS. 3A, 3B and 3C are side, front and top-plan views of a firstalternative embodiment of the invention. In this embodiment, a claddingmaterial 320 having a refractive index greater than air but less thanthe refractive index of the waveguide material 318 surrounds thewaveguide 313 and the micro-cylinder 312. In this exemplary embodiment,waveguide may be a quantum well structure formed from III/V materialsand the cladding material 320 may be a dielectric such as SiO₂, SiN orpolyimide. The addition of the material 320 allows the micro-cylinder312 to be positioned at a greater distance from the waveguide 313. Italso reduces the finesse of the micro-cylinder 312. This reduced finessemay be desirable for a laser tuning device because the reduced finessealso results in an increase in the width of the resonant peaks, allowinglight at wavelengths in a band around the desired wavelength to becoupled to the micro-cylinder 312. The exemplary waveguide 313 includesthe waveguide material 318 positioned on the substrate 110 and acladding layer 316. The exemplary micro-cylinder 312 is also formed fromthe waveguide material 318 and the cladding material 316, and furtherincludes the metal contact 114, described above. In this exemplaryembodiment, the waveguide material 318 may include a single quantumwell, multiple quantum wells or even bulk semiconductor material.Consequently, the resonant wavelength of the micro-cylinder 312 may beadjusted as described above.

[0050] Although not explicitly described below, it is contemplated thatany of the exemplary embodiments of the invention, including theembodiment described above with reference to FIGS. 1A, 1B and 1C may beimplemented with a cladding material 320 which surrounds the waveguideand the resonator, as shown in FIG. 3.

[0051]FIGS. 4A, 4B and 4C are respective side, front and top-plan viewsof another alternative embodiment of the invention. This embodiment isthe same as either of the embodiments described above except that italso includes a power monitor structure 412. The exemplary power monitormicro-structure 412 may formed in the same way as the micro-cylinderresonator 112 or 312 but having a much smaller diameter. A device ofthis type does not resonate in the range of wavelengths that are ofinterest. Because it does not resonate, the power monitor structure mayhave a shape (not shown) other than a disc. For example, it may be asmall rectangular shape. The micro-structure 412 is formed on top of thesubstrate 110 from the waveguide layer 118 and cladding layer 116 andfurther including an electrical contact 414. The electrical contact 414and a further electrical contact (not shown) on the substrate 110 areused to sense a photocurrent or photoresistance in the micro-structure412 which changes in response to light propagating through the waveguide113.

[0052] Due to its small size, the micro-structure 412 does not resonateat any wavelength in the tunable bandwidth of the exemplary waveguide116, either in whispering gallery mode or in modes across the diameteror other dimension of the device. Thus, the photocurrent evanescentlyradially coupled into the micro-structure 412 is proportional to theoptical energy flowing through the waveguide 113 irrespective of itswavelength. Consequently, the photocurrent or photoresistance sensedusing the micro-structure 412 represents the power level of the opticalsignal propagating through the waveguide 113. The embodiment of theinvention described above with reference to FIG. 3 may be especiallyuseful with the addition of micro-structure 412. As described above,although the power detector micro-structure is shown as amicro-cylinder, it may have other shapes, such as rectangular orsemi-circular. Indeed, these shapes may be more advantageous as theyincrease the area through which evanescent coupling may occur, thusincreasing the induced photocurrent in the photodetector of the device.

[0053] Alternatively, the power monitor may be formed from a large disc415 (shown in phantom in FIG. 4C) which exhibits multiple closely spacedresonant modes. Because of its relatively large number of closely spacedresonant modes, this power monitor 415 effectively “resonates” at allwavelengths of interest. This type of power monitor may be moresensitive than the small-dimensioned monitor, described above, and, so,may be more desirable for a given application.

[0054] In the exemplary embodiment of the invention, the power monitormay be used in the control system 214, shown in FIG. 2, to allow thecontroller to distinguish between changes in the optical power signalsensed by the micro-cylinder 112 resulting from a shift in thewavelength of the signal being provided by the waveguide 113 and changesin the power level of the signal propagating through the waveguide 113.

[0055]FIGS. 5A, 5B and 5C are, respectively, side, front and top-planviews of an alternative embodiment of the invention in which themicro-cylinder 512 is formed above a waveguide 513. In this embodimentof the invention, the waveguide 513 is formed on top of the substrate110 and is encapsulated in a cladding material 520. The micro-cylinderresonator 512 is formed on top of the cladding material 520 and, in theexemplary embodiment of the invention, includes an N-layer 517, aintrinsic or quantum well layer 518, a P-layer 516 and a metal contact514. The cladding material 520 may be, for example, an intrinsicsemiconductor material on which the N-type material 517 may be depositedor grown.

[0056] As shown in FIGS. 5A and 5C, the center 524 of the micro-cylinder512 is offset from the center line 522 of the waveguide 513 by adistance d. This separation allows optical signals propagating throughthe waveguide 513 to evanescently couple into the whispering gallerymode of micro-cylinder 512.

[0057]FIGS. 6A, 6B and 6C are side, front and top-plan views of yetanother exemplary embodiment of the invention. This embodiment includesa temperature control element 610 such as a resistive heating element ora TE cooler which is used to adjust the refractive index of thewaveguide material 118. This alternative embodiment also includes atemperature probe 612, to control the operation of the element 610 andan alternative optical power measuring device 614. Otherwise, theapparatus shown in FIGS. 6A, 6B and 6C is identical to the apparatusdescribed above with reference to FIGS. 4A, 4B and 4C.

[0058] In the exemplary embodiment of the invention, the waveguide layer118 includes a single or multiple quantum well structure and anelectrical contact 614 configured across the P-type cladding material116 that covers the waveguide material 118. This metal contact be usedto sense a photocurrent induced by light flowing through the waveguidematerial 118 without significantly affecting the transmission of thelight. Thus, signals between the electrical contact 614 and anelectrical contact (not shown) on the substrate 110 may be used insteadof signals provided by the micro-structure 412 to sense the power of thelight propagating through the waveguide 118.

[0059] The temperature control element 610 may be used to increase ordecrease the temperature of the substrate 110, and thus, the waveguidematerial 118, in order to control the refractive index of the waveguidematerial. If the waveguide material is an optical gain medium for asemiconductor laser, the change in temperature may change the wavelengthof the laser light. The temperature probe 612 is used to sense thetemperature of the substrate to determine if the temperature controlelement may be used to adjust the temperature of the substrate and, ifit may be used, to determine the level of current needed to achieve thedesired adjustment. The exemplary temperature probe 612 is shown asbeing inserted into the bottom surface of the substrate. This is onlyone of several possible positions for the probe. It is contemplated, forexample, that a probe 612′ may be inserted into the side of thesubstrate.

[0060] Furthermore, as shown in FIGS. 6A, 6B and 6C, the TE cooler 610covers much of the bottom surface of the substrate 110. In analternative embodiment of the invention (not shown), where the laserincludes Fabry-Perot or diffractive optical reflectors (not shown), a TEcooler may be implemented locally beneath each reflector to selectivelychange only the refractive index of the reflector in order to change thewavelength of the light produced by the laser.

[0061] Although not explicitly described above or below, it iscontemplated that one or more TE coolers such as that shown in FIGS. 6A,6B and 6C may be used in any of the embodiments of the invention toadjust the refractive index of the devices as desired to control thewavelength of light emitted by the device. Because adjustment of thewavelength by cooling or heating the device is a relatively coarseadjustment which takes some time to complete, it may be desirable to usea combination of temperature and electrical control of the device. TheTE cooler may be used, for example, to implement a coarse change in therefractive index of the device while electrical signals may be used toimplement a small change. Using these two methods, any of the devicesshown in FIGS. 1A-1C and 2A-13C may be controlled to provide light at adesired wavelength.

[0062]FIGS. 7A, 7B and 7C are respective side, front and top-plan viewsof yet another embodiment of the invention. In this embodiment,micro-cylinder structures 712 and 720 are physically coupled to thewaveguide 713 so that leaky-mode coupling is used to transfer light fromthe waveguide 713 to the resonator 712 and the structure 720. As in theprevious examples, the device is formed on a substrate 110 using awaveguide layer 718 and a cladding layer 716. The waveguide 713, thetuning micro-resonator 712 and the power monitor structure 720 are allformed from these three layers. In addition, the structures 712 and 720each include a respective contact layer 714 and 722.

[0063] In operation, light propagating through the waveguide 713scatters into the micro-cylinder structures 712 and 720 at the pointsalong the waveguide 713 which are in contact with the structures 712 and720. Because they contact the waveguide 713, the structures 712 and 720do not have a completely cylindrical shape. Their circularcross-sections are truncated by the edge of the waveguide 713. Thistends to reduce the finesse of the resonator 712, broadening itsspectral peaks. As described below with reference to FIG. 14, this maybe desirable when the monitoring devices 712 is used to tune thewavelength of a laser.

[0064] Because the embodiment of the invention shown in FIGS. 7A, 7B and7C does not rely on evanescent coupling, it may provide greaterphotocurrents than the micro-resonators of the embodiments describedabove and below, that use evanescently coupled resonators. Thisincreased photocurrent may allow the use of relatively low-finesseresonators. It is desirable, however, to form the micro-cylinders 712and 720 in a manner that causes only relatively small amounts ofscattering within the waveguide 713. Excessive scattering may introduceundesirable modes into the waveguide 713 or cause undesirable feedbackinto the laser.

[0065] Another embodiment of the invention that uses scattering topropagate light from the waveguide to the micro-resonators is shown inFIGS. 8A, 8B and 8C. This embodiment employs a waveguide 813 thatincludes small structures 820 formed on one side of the waveguide 813.In the exemplary embodiment of the invention, the structures 820 areformed from the same material as the waveguide layer 818. The structures820 may desirably locally increase the strength of the evanescent fieldand, so, act as antennas, broadcasting optical energy from the waveguide813 to the micro-resonators 114 and 414.

[0066] The device shown in FIG. 8 includes a waveguide 813 formed on thetop of substrate 110 from waveguide layer 818 and cladding layer 116.The micro-resonator 112 and optional power-level micro-structure 412 maybe formed in the same way as described above with reference to FIG. 4.Because of the antenna effect of the structures 820, however, themicro-structures 112 and 412 may be placed at a greater distance fromthe waveguide 813 than the micro-resonators shown in FIG. 4.

[0067] The structures 820 are sources of scattering in the waveguide 813and, consequently, are desirably configured to introduce only a smallamount of scattering to avoid producing undesirable modes in thewaveguide 813.

[0068] The alternative embodiment of the invention shown in FIGS. 9A, 9Band 9C employs multiple micro-cylinder resonators 112, 912, 916 and 920,each of which is tuned to a respectively different wavelength. Theexemplary device also includes a power monitor micro-cylinder 924. Asshown in FIGS. 9B and 9C, each of the micro-cylinders 112, 912, 916, 920and 924 is formed on the substrate 110 from the waveguide material 118and cladding material 116. In addition, each of the micro-cylinders 112,912, 916, 920 and 924 has a respective electrical contact 114, 914, 918,922 and 926. These micro-cylinders operate in the same way as the othermicro-cylinders described above.

[0069] The exemplary micro-cylinders 112, 912, 916 and 920 haverespectively different diameters and are configured to resonate atrespectively different wavelengths that define channels in a DWDMcommunications system. In this embodiment of the invention, each of themicro-cylinders 112, 912, 916 and 920 is a relatively high-finessedevice and the laser is tuned to one of the wavelengths by maximizing apower signal, relative to the power signal provided by the power-metermicro-cylinder 924, of a respective one of the resonators 112, 912, 916and 920. As described above, the electrical contacts 114, 914, 918 and922 both allow the photocurrent to be sensed and allow the respectivemicro-resonator to be tuned by application of a bias current. Becauseeach micro-resonator has a different curvatures, it may also have adifferent finesse. This different finesse may be compensated byindividually tuning the indices of refraction of the micro-resonatorselectronically. Injecting current into the device, however, also affectsthe losses exhibited by the device.

[0070]FIGS. 10A, 10B and 10C represent side, front and top-plan views ofanother embodiment of the invention. This embodiment employs amicro-resonator 1012 having an oval cross-section. The ovalcross-section reduces the finesse of the resonator, increasing thespectral range of its resonant peaks. As described below, amicro-resonator having lower Q may be desirable when the micro-resonatoris used to adjust the wavelength of a semiconductor laser. Themicro-resonator 1012 is formed from oval sections of the waveguidematerial 118 and cladding material 116. It may also include an ovalelectrical contact 1014. The remainder of the device shown in FIGS. 10A,10B and 10C is formed and operates in the same way as described abovewith reference to FIG. 4. Although not shown, it is contemplated thatmirco-resonator 1015 (shown in phantom) having a stadium shape may alsobe used. Alternatively, a micro-resonator 1017 (shown in phantom) havingan octagonal shape may be used. In for the resonators 1015 and 1017, thestraight sides of the shapes are desirably positioned parallel to thewaveguide to increase coupling between the waveguide and the resonator.Although not shown, it is contemplated that a hexagon-shapedmicro-resonator may also be used. For the hexagon and octagon-shapedmicro-resonators, the dominant resonant mode traces a path around theedge of the shape, reflecting from the sides on the ends of the device.

[0071] Another exemplary embodiment of the invention is shown in FIGS.11A, 11B and 11C. This embodiment employs a disc instead of amicro-cylinder as the micro-resonator. The described exemplaryembodiment of the invention includes a micro-cylinder power-monitor1120. In this embodiment, the disc 1121 may be formed from the waveguidematerial 1118 on top of a pedestal 1117 that may be formed from anN-type material 1119 on the N-type substrate 110. The exemplary pedestal1117 may be formed at the same time as an N-type sub-layer for thewaveguide 1113. The N-type material 1119 desirably has an index ofrefraction than is no greater than that of the substrate 110. Thepedestal is patterned to have a smaller cross-section than the disc1121. Next, an optional layer of low-index dielectric or intrinsicmaterial 1123, having a lower refractive index than the waveguide layer1118 is deposited and the device is polished to expose the top surfaceof the pedestal 1117 and the sub-layer on which the waveguide is formed.Next, the disc 1121 and waveguide are formed from the waveguide material1118, as shown in FIGS. 11A through 11C. At the next step, the claddinglayer 1116 is formed on top of the waveguide layer 1118 and electricalcontacts 1122 and 1114 are formed to complete the power monitormicro-cylinder 1120 and the disc micro-resonator 1112.

[0072] As an alternative to forming the pedestal to have a smallercross-section than the disc, it is contemplated that the pedestal may beformed using a plane-selective etchant that undermines the edges of thedisc to form a pedestal structure beneath the disc.

[0073] A micro-resonator operating in whispering gallery mode propagateslight in a ring around the outer surface of the micro-resonator. Thus,the described disc micro-resonator has greater finesse than themicro-cylinder resonators, described above, because the outer portion ofthe disc has a higher difference in its refractive index from thematerial below it than in the previous embodiments.

[0074] Although the exemplary embodiment is shown as having a layer ofintrinsic material 1123 below the outer portion of the disc resonator,it is contemplated that the material 1123 may be, instead, a sacrificiallayer that is removed some time after the waveguide material has beendeposited. In this alternative embodiment, the material 1123 may be, forexample, SiO₂ which is removed using an HF etchant after forming thecladding layer but before forming the metal contacts 1114 and 1122.

[0075] In addition, although the exemplary embodiment shows the disc1121 being separated from the waveguide 1113, it is contemplated that itmay be physically coupled to the waveguide 1113 in the same way that themicro-resonator 712 is coupled to the waveguide 713, as shown in FIG. 7.In this configuration the disc 1121 receives light from the waveguide1113 via leaky-mode coupling.

[0076] In another alternative embodiment of the invention, shown inFIGS. 12A, 12B and 12C, the micro-resonator is a ring resonator 1212,having a central portion 1215 that has a lower refractive index than theouter portion 1217. This device may be formed in the same way as thedevice shown in FIG. 4 except that the micro-cylinder 1212 is formedfrom a hollow cylinder 1218 of the waveguide material 118 and a hollowcylinder 1216 of the cladding material 116. The resonator 1212 and iscapped with a hollow cylinder 1214 metallization. The power monitormicro-structure 412 and the waveguide 113 are formed in the same way asdescribed above with reference to FIG. 4.

[0077] The device shown in FIGS. 12A, 12B and 12C may have advantagesover the devices which use micro-cylinder or disc resonators because thering resonator tends to prevent modes that reflect across arcs ofresonator, that is to say, modes other than the whispering gallery modesand, thus, may be more sensitive.

[0078] Although the resonator 1212 is shown as a hollow cylinder, it iscontemplated that it may be filled, for example, with a dielectricmaterial, such as the material 320 shown in FIG. 3. As in FIG. 3, thismaterial may also surround both the waveguide 113 and themicro-resonator 1212.

[0079] In addition, although the exemplary embodiment shows the ringresonator 1212 being separated from the waveguide 113, it iscontemplated that it may be physically coupled to the waveguide 113 inthe same way that the micro-resonator 712 is coupled to the waveguide713, as shown in FIG. 7. In this configuration the ring resonator 1212receives light from the waveguide 113 via leaky-mode coupling.

[0080]FIGS. 13A, 13B and 13C illustrate yet another exemplary embodimentof the invention, including two micro-resonator structures 1312 and1320. These structures have different diameters and, so, resonate atdifferent wavelengths. Each of the two resonators is formed on thesubstrate 110 from discs of the waveguide material 118 and claddingmaterial 116. Each resonator 1312 and 1320 is capped with a respectivecontact 1314 and 1322.

[0081] Rather than tuning each resonator to a different channel of theDWDM system, as described above with reference to FIGS. 9A, 9B and 9C,however, the embodiment shown in FIGS. 13A-13C forms the two resonatorsso that, near the desired wavelength band, the resonance peak of onemicro-resonator occurs at approximately the same wavelength as themidpoint in the slope of the other micro-resonator.

[0082] This may be described more easily with reference to FIG. 14. FIG.14 is a graph of amplitude versus wavelength that illustrates theresponse of the two micro-resonators 1312 and 1320, shown in FIGS.13A-13C. In FIG. 14, graph 1410 represents the response of the resonator1312 and graph 1412 represents the response of resonator 1420. As shownby the line 1414, at the wavelength of interest, λ₀, the peak in theresponse of micro-resonator 1312 corresponds to the mid-point in theslope of the response of resonator 1320. U.S. Pat. No. 6,323,987entitled, CONTROLLED MULTI-WAVELENGTH ETALON, describes a method oftuning a laser using the slope of a response curve. This method isdesirable because relatively small changes in the frequency of the laserlight can be sensed and corrected.

[0083] By designing the micro-resonators 1314 and 1320 to have relativesizes and free-spectral ranges such that, at least over a few cycles ofthe response curve, the successive slopes of the resonators occur atwavelengths that represent successive channels in the DWDM system, asingle laser may be tuned over multiple channels within that range. Forexample, as shown in FIG. 14, if λ₀ represents one DWDM channel and λ₁represents the next channel, then the laser may be tuned to λ₀ on thepositive-going slope of the response curve 1412 and to λ₁ on thenegative-going slope of the response curve 1410.

[0084] Because the two micro-resonators have different diameters, theyhave different resonant wavelengths and different free spectral ranges.Consequently, the effect, described above, in which the peak of oneresponse curve coincides with the slope of the other response curve isonly maintained over a certain range of wavelengths. As shown in FIG.14, for example, the alignment between the peaks and slopes of thecurves 1410 and 1412 is less desirable at wavelengths λ_(N) and λ_(N+1)indicated by the lines 1414′ and 1416′ than it is at λ0 and λ1. Thespacing between the wavelengths λ_(N) and λ_(N+1) does not correspond tothe spacing between adjacent DWDM channels. Thus, the device shown inFIG. 13A through 13C may be effectively used only over definite range ofwavelengths, which may be less than the full span of a DWDM system.

[0085] The range over which a system may be used, however, may beexpanded by using more than two micro-resonators, as shown in FIG. 9, orby forming the two micro-resonators to have free-spectral ranges thatare approximately matched by using larger micro-resonators operating onhigher mode whispering gallery modes.

[0086] While the invention has been described in terms of exemplaryembodiments, it is contemplated that it may be practiced as describedabove within the scope of the appended claims. For example, although theexemplary embodiments are shown as using disc micro-resonators thatoperate in whispering gallery modes, it is contemplated that other typesof resonators, operating in other modes may be used instead. Forexample, rectangular resonators having a resonant length between theirends may be used in place of the disc resonators.

What is claimed:
 1. A tunable semiconductor laser system comprising: asemiconductor laser, formed on a semiconductor substrate, including anoptical gain medium having an index of refraction and an optical pathlength, the laser producing laser light at a predetermined wavelength;tuning means, coupled to the substrate and responsive to a tuning signalto adjust one of the index of refraction and the optical path length ofthe optical gain medium to change the predetermined wavelength of thelaser light; a micro-resonator structure formed on the semiconductorsubstrate and optically coupled to the optical gain medium and includingan optical sensor configured to provide an electrical output signal,wherein the micro-resonator is configured to resonate when light at apredetermined wavelength propagates through the optical gain medium andto provide the electrical output signal having a peak value in responseto light at the predetermined wavelength; and control circuitry coupledthe micro-resonator and to the tuning means for providing the tuningsignal in response to the output signal of the micro-resonatorstructure.
 2. A tunable semiconductor laser system according to claim 1,wherein the micro-resonator is a micro-cylinder.
 3. A tunablesemiconductor laser system according to claim 1, wherein themicro-resonator is positioned with respect to the optical gain medium sothat light propagating through the optical gain medium is received bythe micro-resonator via evanescent coupling.
 4. A tunable semiconductorlaser system according to claim 3, wherein each of the optical gainmedium and the micro-resonator has a refractive index and the devicefurther includes a cladding material positioned between the optical gainmedium and the micro-resonator, the cladding material having anrefractive index less than the refractive index of both the optical gainmedium and the micro-resonator.
 5. A tunable semiconductor laser systemaccording to claim 1, wherein the micro-resonator is physically coupledto the optical gain medium so that light propagating through the opticalgain medium is received by the micro-resonator through leaky-modecoupling.
 6. A tunable semiconductor laser system according to claim 1,further including circuitry for applying a bias signal to themicro-resonator to cause the micro-resonator to resonate at a differentwavelength.
 7. A tunable semiconductor laser system according to claim1, further including means, for measuring an amount of optical powerpropagating through the optical gain medium to provide a power levelsignal.
 8. A tunable semiconductor laser system according to claim 7,wherein the means for measuring comprises a micro-structure, having aresonant length less than any wavelength of light that propagatesthrough the optical gain medium.
 9. A tunable semiconductor laser systemaccording to claim 8, wherein the micro-structure is positioned withrespect to the optical gain medium so that light propagating through theoptical gain medium is received by the micro-structure via evanescentcoupling.
 10. A tunable semiconductor laser according to claim 8,wherein the micro-structure is physically coupled to the optical gainmedium so that light propagating through the optical gain medium isreceived by the micro-structure through leaky-mode coupling.
 11. Atunable semiconductor laser according to claim 7, wherein the means formeasuring comprises a micro-structure having a plurality of resonantmodes such that light having wavelengths in a range of interest for thetunable semiconductor laser resonates in the micro-structure.